This application claims the benefit of German Application No. 102 28 868.2 filed Jun. 27, 2002.
The invention concerns a device to generate mechanical work with a steam engine that operates with a closed circuit and that has a feed water tank, a feed pump, an evaporator to generate steam, the steam engine, and a condenser.
There are prior art devices of this kind in which the evaporator is heated by hot exhaust from a burner. This evaporator is supplied feed water through the feed pump from a feed water tank. The feed water is evaporated and superheated. This superheated steam is fed to a steam engine. In a prior art device of this kind, the steam engine is a rotary piston engine in the form of a vane motor. The expanded steam leaving the steam engine is condensed in a condenser. The condensed water is then re-supplied to the feed water tank. This device therefore operates in a closed steam/water circuit. The advantage of such devices is that they release very few pollutants with suitable burners.
Areas of application for such devices that simultaneously generate heat and mechanical work can be auxiliary power units that provide heat or power for auxiliary heating or any power consumer, for example when the prime mover of a vehicle is not operating (passenger car, truck, trailer, boat, yacht, etc.).
A problem with such mobile units, especially those that use water as the working fluid, is frost protection. Such units are sometimes used at low environmental temperatures. It must therefore be ensured that the different parts of the closed circuit are not damaged when the water in the closed circuit freezes while the unit is not operating.
In vehicles with internal combustion engines, the coolant water is kept from freezing by adding antifreeze. This antifreeze lowers the freezing point of the coolant water. The coolant water is not a working fluid of that kind of engine. The coolant water remains fluid in the secondary coolant circuit while the internal combustion engine is operating. The temperature remains under 100xc2x0 C./212xc2x0 F. At these temperatures, conventional antifreezes remain stable. However, water is the operating medium of a steam engine. The water is evaporated. Temperatures of up to 900xc2x0 C./1650xc2x0 F. may arise. At such temperatures, conventional antifreezes decompose. In addition, the water or steam flow as the operating medium through the active parts of the device, evaporator and steam engine. Problems with corrosion and deposits can arise.
It is prior art to drain pipe systems that can freeze, such as garden pipes in the winter. It is however not practical to drain the water from the system of a device of the above-cited kind. The system would have to be refilled with water each time it was started. This would negate the advantage of the closed circuit. Fresh water contains minerals that form scale deposits when the water evaporates.
The invention is based on the problem of keeping a device of the initially cited kind free of damage from freezing water when the device is not operating under low environmental temperatures.
The invention solves this problem in that
(a) the feed water tank has an inert gas area above the feed water
(b) the feed water tank is designed to be frost-resistant
(c) a valve arrangement is provided that can switch the device to a state in which the feed water is expelled from at least the frost-sensitive parts of the circuit by inert gas and moved into the feed water tank
According to the invention, an inert gas in the feed water tank is used to expel the feed water from the other part of the circuit to the feed water tank. This is done by switching a valve arrangement. The feed water tank is designed so that it will not be damaged, for example by exploding, from freezing water inside. The advantages of the closed circuit are retained. To restart the device, the system is reheated, and the valve arrangement only has to be switched back to the position suitable for normal operation.
A situation may arise in which residual water remaining in parts of the circuit after expulsion freezes and causes damage as it expands. In another embodiment of the invention, it is therefore provided that the inert gas is a substance such as xenon that forms a gas hydrate with water.
There are gaseous substances that do not dissolve in water but form a gas hydrate with water at low temperatures. Gas hydrates are not genuine compounds. The gas such as xenon is instead enclosed by the water molecules in an ice-like structure. It has been demonstrated that such frozen gas hydrates exert substantially less pressure on surrounding walls than normal ice. Even relatively weak structures such as glass tubes can withstand the pressure exerted by freezing gas hydrates. By using a substance as the inert gas that forms such a gas hydrate which also expels water from the system under pressure, the remaining water in the system forms a gas hydrate whose pressure does not damage the parts of the circuit upon freezing.
In the following description, a switching sequence is also described for the otherwise unchanged valve arrangement in which the water is not expelled from the circuit into the feed water tank but rather the water is only enriched with the inert gas forming the gas hydrate.
In an embodiment of the invention, the feed water tank is connectable via the valve arrangement with a variable volume feed water reservoir. In a first valve state of the valve arrangement, the circuit that runs from the feed water tank via the feed pump, the evaporator, steam engine and the condenser back to the feed water tank is closed, In a second valve state of the valve arrangement, the water from the reservoir can be fed to the feed water tank by the feed pump to generate overpressure from the inert gas in the system comprising the evaporator, steam engine and the condenser. In a third valve state of the valve arrangement, the feed water tank is separated from the circuit and connected to the reservoir to release pressure, and in a fourth valve state of the valve arrangement, the system under pressure is connected with the de-pressurized feed water tank. An overpressure can be maintained in the system in a fifth valve state of the valve arrangement. The circuit has a first pipe section between the part of the feed water tank filled with feed water and the feed pump in which ends a connecting line to the reservoir with a changeable volume. The circuit has a second pipe section between the feed pump and the evaporator. The circuit has a third pipe section between the condenser and the inert gas area filled with inert gas above the water surface of the feed water tank. A fourth pipe section extends between the inert gas area of the feed water tank and the second pipe section. The valve arrangement has a first and second controllable in-line valve that are in the first pipe section, whereby the connecting line ends at the reservoir between these valves. The valve arrangement has a third and fourth controllable in-line valve that are in the second pipe section, whereby the fourth pipe section is connected between these valves to the second pipe section. The valve arrangement has a fifth valve that is in the fourth pipe section. The valve arrangement has a sixth valve that is in the third pipe section. Finally, the valve arrangement has a seventh valve that is in the connecting line to the reservoir with a variable volume. In the first valve state of the valve arrangement, the first, second, third, fourth, and sixth valves are open, and the other valves are closed. In the second valve state of the valve arrangement, the second, third, fifth and sixth valves are open, and the other valves are closed. In the third valve state of the valve arrangement, the first and seventh valves are open, and the other valves are closed, and in the fourth valve state of the valve arrangement, the fourth and fifth valves are open, and the other valves are closed.
Exemplary embodiments of the invention are further explained below with reference to the associated drawings.